1. Field of the Invention
The present invention relates to a process for producing a powder of a transition-metal boride which has prominent characteristics such as high hardness, a high melting point, high corrosion resistance and good electrical conductivity.
2. Description of the Related Art
A transition-metal boride such as titanium diboride, tantalum diboride or the like is utilized in, for example, wear-resistant materials, corrosion-resistant materials, electrical contact materials and the like.
As an example of an industrial process for producing a powder of titanium diboride or tantalum diboride, there is a process in which a powder mixture of metal titanium and boron or a powder mixture of metal tantalum and boron is reacted with heating, a process in which a mixture of titanium oxide, boron oxide and carbon or a mixture of tantalum pentoxide, boron oxide and carbon is carbothermically reduced at a temperature around 1000.degree. C., or a process in which a mixture of metal titanium, boron carbide and carbon or a mixture of metal tantalum, boron carbide and carbon is reacted at a high temperature around 2000.degree. C.
However, since the powders of transition-metal boride such as titanium diboride produced by the above processes contain agglomerated large secondary particles formed by firmly attaching of primary particles, a grinding step has been required in order to obtain powders therefrom which have a desired particle size, for example, below 10 .mu.m. However, it is difficult to carry out the grinding step since the transition-metal boride powder has a extremely high hardness.
Accordingly, in order to solve the above described problem, the following processes for the producing powders of transition-metal boride such as titanium diboride have been proposed.
As one process, it is known that a single crystal of a transition-metal boride such as titanium diboride is produced in a metal flux.
This process includes reacting a mixture of metal titanium, crystalline boron powder and aluminum chips as a metal flux under an argon atmosphere at a temperature in a range from 1000.degree. to 1600.degree. C. to produce the single crystal of titanium diboride, and the process is described in Research Report of Kanagawa University, Technology Faculty (No. 23, March, 1985). The single crystal obtained by the above process is a single crystal in the form of a thin plate-like (platelet) form of which the size is around 5 .mu.m at a reaction temperature in a range from 1000.degree. to 1300.degree. C., and a large single crystal particle in the form of a hexagonal polyhedron of which the size is around 15 to 20 .mu.m at a reaction temperature in a range from 1400.degree. to 1500.degree. C.
Another process is described in Bulletin of the Chemical Society of Japan, No. 8, page 1535 (1985) in which a mixture of metal tantalum, crystalline boron powder and aluminum chips as a metal flux is reacted under an argon atmosphere at a temperature in a range from 1150.degree. to 1500.degree. C. to produce single crystal of tantalum diboride. The single crystal obtained by this process is a single crystal particle in the form of a hexagonal polyhedron of which size is a few micrometers at a reaction temperature in a range from 1150.degree. to 1400.degree. C. and around 10 to 15 .mu.m at a reaction temperature in a range from 1400.degree. to 1500.degree. C.
In the above processes, metal titanium or metal tantalum dissolves in the metal flux and gradually reacts with boron to produce single crystal particles of the transition-metal boride, and bonds between the obtained single crystals are weak so that a degree of agglomeration of the crystal particles is low. However, since the amount of boron dissolved in the metal flux is extremely small, unreacted boron tends to remain at a reaction temperature less than 1000.degree. C. Therefore, a high temperature not lower than 1000.degree. C. is required for the sufficient reaction.
In addition, the boron powder which is one of the feed materials to be used by addition to the metal flux is so expensive that the above processes can not be industrially efficient processes.
Then, a process was developed, in which a molten salt is used as a flux for dissolving a titanium compound and a boron compound to produce a powder of titanium diboride.
This process produces a titanium diboride powder by adding K.sub.2 TiF.sub.8 and KBF.sub.4 to molten salts of, for example, LiF--KF or KF--KCl, dissolving and electrolyzing them, which is disclosed in METALL, vol. 42, 1196 (1988). According to this process, the titanium diboride powder having a particle size in a range from 0.2 to 7 .mu.m may be obtained at a reaction temperature of around 800.degree. C. Not only does the addition and the dissolution of the titanium compound and the boron compound into the molten salts fail to produce the titanium diboride, but the electrolysis is also essential. Accordingly, this process can not be said to be an industrially efficient process.
Those conventional processes as described above have some disadvantages such as the necessity of expensive feed materials and the necessity of high reaction temperatures above 1000.degree. C.